This invention concerns a key-touch sensor and method of manufacture therefor. More specifically, this invention relates to a key-touch sensor having output voltage characteristics as a function of the load upon touching the keyboard and the method of manufacturing the same. The key-touch sensor of the invention has a novel structure, shows less variations in the load-output voltage characteristics and exhibits good reproducibility. It is suited to mass production with reduced thickness and in an integrated form.
Various kinds of switches using pressure-sensitive electroconductive rubber sheets for input elements have generally been used as electronic parts. Examples of input switches utilizing pressure-sensitive electroconductive rubber sheet, include ordinary ON-OFF switches, as well as input elements in a device for detecting manuscription input positions. Further, a key-touch sensor for use in electronic musical organs using pressure-sensitive electroconductive rubber sheets (as disclosed in Japanese Patent Publication Kokai Nos. 53-79937 and 54-80350) has been put to practical use by applying the property of the pressure-sensitive electroconductive rubber as the pressure-sensitive sensor. The sensor or keyboard has a structure in which a ribbon of a pressure-sensitive electroconductive rubber sheet sandwiched between metal sheets as electrodes is placed in a shallow groove disposed in an extruded material made of polyvinyl chloride resin and covered from above with felt. After depressing a key for producing a sound, when further pressure is applied resistance change is caused to the sensor, which is converted into an electric signal between terminals by a constant current and outputted as a predetermined control signal from an operational amplifier. Volume, tone or pitch of the sound may be controlled in this manner.
However, key-touch sensors using the pressure-sensitive electroconductive rubber sheet have a drawback in that the load-output voltage characteristics vary and the reproducibility thereof is not satisfactory. Accordingly, a sensor of this type cannot be used as a key-touch sensor requiring high accuracy. Further, in the case of manufacturing the key-touch sensor, the number of production steps is great due to the complicated shape. No satisfactory mass production can be obtained by the existent molding process since an extremely long ribbon-like sheet is required. The sensor manufactured by the method of the prior art also has the drawbacks that the thickness is large, setting to the electrode is difficult and the sensor is expensive.
F. N. Eventoff discloses, in U.S. Pat. Nos. 4,268,815, 4,314,277 and 4,315,238, a pressure-sensitive switch or pressure transducer, in which the first and second contacts are usually placed opposing each other and the opposing faces are brought into contact to establish electrical conduction by an external pressure through a pressure-sensitive layer and/or semiconductor layer. One of the features of these patented inventions resides in the fact that at least one of the layers put between the contacts is constituted with a layer containing semiconductive particles such as molybdenum sulfide particles. However, the structure having such a semiconductor layer has draw-backs in that the pressure-response varies and response changes with aging. Also, the anisotropy leads to restriction in the application use.